Increasing storage capacity is currently one of the priorities of research and development of the secondary battery. Among secondary batteries, lithium ion battery that use materials capable of inserting/extracting lithium ions as positive electrode and negative electrode respectively shows the highest energy density. However, due to the limited capacities of the cathode and anode materials, the highest energy density that may be achieved by lithium ion battery is limited by above materials and may not be increased further. Therefore, the development of an electrode material with high capacity is urgently needed.
Among the plurality of materials, active metals such as lithium, sodium, magnesium, calcium, and aluminum used as the active material of battery have the advantages of lightweight and high capacity. In particular, lithium metal and materials that capable of alloying with lithium (such as silicon, tin, and aluminum) may all achieve the effect of high capacity. However, the volumes of materials that capable of alloying with lithium inevitably expand when reacting with lithium, which causes the active material to fragment and peel when charged and discharged repeatedly, and consequently it causes poor cycle life thereof. Among active metals, for instance, although lithium metal has a capacity reaching 3862 mAh/g, lithium metal is very active. Lithium metal is not only sensitive to moisture and air but also capable of reacting with species in the electrolyte solution during charge and discharge process, which leads to the loss of activity of lithium metal and the reduction in the capacity provided by lithium metal. Other active metals such as sodium, magnesium, calcium, and aluminum may also be corroded by outside reactive materials, which affect the capacity thereof. On the other hand, for instance, dendritic lithium deposits may be produced on the surface of the lithium metal after being charged and discharged repeatedly. A hidden safety concern of the lithium deposit with the particular surface is that the separator may be punctured and cause a short circuit. Therefore, if an electrically-conductive protection layer is be coated on the surface of the active metal electrode, then the capacity of the device may be effectively increased and the cycle life thereof may also be increased.
Currently, the protection layer used as the surface of the active metal electrode may be a single-layer structure or a multi-layer structure. If the surface protection layer uses a single-layer structure design, then the deterioration of active electrode may not been effectively suppressed during the operation of device. However, if the surface protection layer uses a multi-layer design, then problems of compatibility and electrical conductivity occur. In recent technology, if the multi-layer structure includes ion-conducting ceramic, ion-conducting salt, organic compound, polymer and such, then the problem of poor electrical conductivity may readily occur. If the multi-layer structure includes a metal that may conduct an alloy reaction with ions, then the volume of the multi-layer structure may inevitably expand when alloyed with the ions, therefore causing each electrode layer to be unable to maintain a stable compatible structure, which in turn affects service life. Fragmentation of the protection layer material may even occur.